Phosphorylation of glycogen synthase kinase-3 β at serine 9 confers cisplatin resistance in ovarian cancer cells

Cisplatin is commonly used in the treatment of advanced ovarian carcinoma. A major limitation of the use of cisplatin is the development of resistance in tumors. Glycogen synthase kinase-3 ss (GSK-3ss) is a multi-functional serine/threonine kinase. Its activity is regulated negatively by the phosphorylation of serine 9 (pGSK-3ss-ser-9) and positively by the phosphorylation of tyrosine 216 (pGSK-3ss-tyr-216). We compared the expression/phosphorylation of GSK-3ss between the cisplatin-sensitive ovarian carcinoma cell line A2780 and its cisplatin-resistant derivative CP70. The expression levels of total GSK-3ss and pGSK-3ss-tyr-216 were similar in these cells; however, CP70 cells had a much higher expression of pGSK-3ss-ser-9 than A2780 cells. Lithium chloride, which is a GSK-3ss inhibitor and stimulates pGSK-3ss-ser-9, significantly increased the IC50 of cisplatin and counteracted cisplatin-induced apoptosis of A2780 and CP70 cells. In contrast, overexpression of a constitutively active S9A GSK-3ss mutant increased the sensitivity of CP70 cells to cisplatin and significantly enhanced cisplatin-mediated apoptosis. It is suggested that the cisplatin-resistance of CP70 cells is mediated by stabilizing p53. We demonstrated that GSK-3ss negatively regulated the expression of p53. Therefore, pGSK-3ss-ser-9 may confer the cisplatin resistance of ovarian carcinomas through the stabilization of p53 expression. Our study establishes a potential role of GSK-3ss in the development of cisplatin resistance in initially sensitive tumors.